This invention relates generally to coated web drying systems, and is concerned in particular with an improved method and apparatus for controlling the operation of convective air dryers employed in such systems.
A conventional flotation-type convective air dryer is diagrammatically illustrated at 10 in FIG. 1. The dryer comprises an insulated housing 11 defining a drying chamber 12 with communicating entry and exit slots 14, 16. A coated paper web 18 enters the housing through slot 14, passes through the drying chamber 12, and exits the housing through slot 16. Heated air is delivered to supply ducts 20 within the dryer at locations above and below the web 18. Each supply duct communicates with a plurality of headers 22 which in turn communicate with nozzles indicated typically at 24. The nozzles apply heated drying air to opposite sides of the web 18. The air picks up moisture evaporating from the web before exiting from the housing via exhaust ducts 26. From here, the moisture laden air is collected, partially diverted and exhausted, and partially recycled, with the recycled air being reheated before being returned to the dryer via the supply ducts 20.
The staggered arrangement of the nozzles induces a sinusoidal-like wave shape to the web as it passes through the dryer. This provides a measure of cross-machine rigidity which flattens mild ripples and enables the web to resist edge curl.
The dryer 10 of FIG. 1 is typically associated with an external air system of the type illustrated diagrammatically in FIG. 2. The system includes a burner chamber 28 or other like heat generator in which combustion air (M.sub.ca) and fuel (M.sub.F) are admitted for combustion. Heated drying air is withdrawn from chamber 28 by a recirculation fan 30 and is directed via conduits 32, 34 to the supply ducts 20. Moisture laden return air is carried from the exhaust ducts 26 back to the chamber 28 via conduits 36, 38. An air exhaust fan 40 communicates with conduit 38 via conduit 42 and serves to divert and remove exhaust air (M.sub.E) from the system. Makeup air (M.sub.MU) is added to the combustion chamber via conduit 44. Flow control devices such as for example dampers 46, 48, 50 respectively control the flow rates of makeup air, drying air and exhaust air.
If the air pressure within the dryer (commonly referred to as "box pressure") is allowed to exceed ambient air pressure, hot air will exfiltrate or "puff" from the dryer through the entry and exit slots 14, 16. Conversely, if box pressure is allowed to drop below ambient air pressure, cold air will infiltrate into the dryer through the slots 14, 16. Infiltration or exfiltration of air is designated at M.sub.I, whereas moisture being evaporated from the web is shown at M.sub.W. The dryer is considered to be "balanced" when there is no infiltration or exfiltration of air through the entry and exit slots.
Exfiltration produces an unacceptable discharge of hot process air into the work environment. This condition is easily recognizable, and often remedied by purposely depressing box pressure to induce infiltration. However, infiltration also results in serious drawbacks, including wasted fuel, loss of dryer capacity and degradation of paper quality. In the past, those skilled in the art either have misunderstood the negative consequences of infiltration, or have chosen to accept them as necessary corollaries to the avoidance of exfiltration.
Maintaining dryer balance requires a carefully coordinated adjustment of both the exhaust and makeup air dampers. The majority of prior art installations do not lend themselves to this level of sophistication. Often, the dampers are manually adjustable, and not readily accessible, thus discouraging operating personnel from achieving and maintaining optimum settings.
During the last decade, some effort has been directed to automating control of the makeup air and exhaust dampers. For example, in U.S. Pat. No. 4,591,517 (Whipple et al), one control loop automatically adjusts the setting of the makeup air damper in response to fluctuations of box pressure above and below ambient air pressure. A second control loop adjusts the setting of the exhaust damper in response to changes in another process variable, e.g., the amount of ink or other liquid being applied to the web. However, because these control loops are not integrated one with the other, the possibility exists that one or the other of the dampers may be adjusted to a fully open or fully closed position. As will hereinafter be described in greater detail, when this occurs, the air system is no longer in control, which in turn means that the dryer is likely to drift into an unbalanced condition.
An objective of the present invention is to avoid the drawbacks of the prior art by providing an improved method and apparatus for continuously and automatically maintaining the dryer in a balanced state.
A companion objective of the present invention is to coordinate adjustments to flow control devices such as the makeup air and exhaust dampers in a manner which avoids either damper from being adjusted to an extreme setting, e.g., fully open or fully closed.
Another objective of the present invention is the provision of a method and apparatus for automatically maintaining the dryer in a balanced state while at the same time automatically controlling other process variables, e.g., fuel consumption, web temperature, etc.
Still another objective of the present invention is the provision of a method and control system for automatically readjusting process set points that might otherwise require the makeup air and/or exhaust air flow control devices to be adjusted to extreme settings, or would require such flow control devices to be adjusted such that preselected high and low limits of another process variable would be exceeded.
Another objective of the present invention is the provision of a method and control system for overriding existing settings of makeup air and/or exhaust flow control devices and forcing such devices to different settings when operating in non-drying modes, e.g., during purge cycles.
These and other objects and advantages of the present invention will now be described in greater detail with further reference to the accompanying drawings, wherein: